Rough engine idle, surging at cruise ??

[Project91z28]

W-bodys are notorious for faulty coil packs. These are the black dual tower pieces the spark plug wires come from. This has caused exactly the symptoms you described.

[wolfy950]

I replaced the EGR Valve today, taking extra care to make sure the gasket was sealed properly. Fired it up, and it STILL stumbles and surges. What else could it be? Maybe this EGR Valve is bad too? TPS? O2 Sensor? Clogged Cat. Converter? Would any of these do this too? It's getting colder by the day, and the colder it gets, the worse it shudders on a cold start.

 

Any error codes? I was getting crazy error codes, bad starting, dieing, hard to keep running, surging at idle, bucking on the highway so I changed out the ECM. Wow! What a difference. Solved everything.